Network Working Group M. Handley, Ed.
Request for Comments: 4732 UCL
Category: Informational E. Rescorla, Ed.
Network Resonance
Internet Architecture Board
IAB
November 2006
Internet Denial-of-Service Considerations
Status of This Memo
This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
Copyright Notice
Copyright (C) The IETF Trust (2006).
Abstract
This document provides an overview of possible avenues for denial-
of-service (DoS) attack on Internet systems. The aim is to encourage
protocol designers and network engineers towards designs that are
more robust. We discuss partial solutions that reduce the
effectiveness of attacks, and how some solutions might inadvertently
open up alternative vulnerabilities.
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RFC 4732 DoS Considerations November 20066. Security Considerations ........................................317. Acknowledgements ...............................................318. Normative References ...........................................319. Informative References .........................................32Appendix A. IAB Members at the Time of This Writing ...............361. Introduction
A Denial-of-Service (DoS) attack is an attack in which one or more
machines target a victim and attempt to prevent the victim from doing
useful work. The victim can be a network server, client or router, a
network link or an entire network, an individual Internet user or a
company doing business using the Internet, an Internet Service
Provider (ISP), country, or any combination of or variant on these.
Denial-of-service attacks may involve gaining unauthorized access to
network or computing resources, but for the most part in this
document we focus on the cases where the denial-of-service attack
itself does not involve a compromise of the victim's computing
facilities.
Because of the closed context of the original ARPANET and NSFNet, no
consideration was given to denial-of-service attacks in the original
Internet Architecture. As a result, almost all Internet services are
vulnerable to denial-of-service attacks of sufficient scale. In most
cases, sufficient scale can be achieved by compromising enough end-
hosts (typically using a virus or worm) or routers, and using those
compromised hosts to perpetrate the attack. Such an attack is known
as a Distributed Denial-of-Service (DDoS) attack. However, there are
also many cases where a single well-connected end-system can
perpetrate a successful DoS attack.
This document is intended to serve several purposes:
o To highlight possible avenues for attack, and by so doing encourage
protocol designers and network engineers towards designs that are
more robust.
o To discuss partial solutions that reduce the effectiveness of
attacks.
o To highlight how some partial solutions can be taken advantage of
by attackers to perpetrate alternative attacks.
This last point appears to be a recurrent theme in DoS, and
highlights the lack of proper architectural solutions. It is our
hope that this document will help initiate informed debate about
future architectural solutions that might be feasible and cost-
effective for deployment.
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In addition, it is our hope that this document will spur discussion
leading to architectural solutions that reduce the susceptibility of
all Internet systems to denial-of-service attacks.
We note that in principle it is not possible to distinguish between a
sufficiently subtle DoS attack and a flash crowd (where unexpected
heavy but non-malicious traffic has the same effect as a DoS attack).
Whilst this is true, such malicious attacks are usually more
expensive to launch than many of the crude attacks that have been
seen to date. Thus, defending against DoS is not about preventing
all possible attacks, but rather is largely a question of raising the
bar sufficiently high for malicious traffic.
However, it is also important to note that not all DoS problems are
malicious. Failed links, flash crowds, misconfigured bots, and
numerous other causes can result in resource exhaustion problems, and
so the overall goal should be to be robust to all forms of overload.
2. An Overview of Denial-of-Service Threats
In this section, we will discuss a wide range of possible DoS
attacks. This list cannot be exhaustive, but the intent is to
provide a good overview of the spectrum of possibilities that need to
be defended against.
We do not provide descriptions of any attacks that are not already
publicly well documented.
2.1. DoS Attacks on End-Systems
We first discuss attacks on end-systems. An end-system in this
context is typically a PC or network server, but it can also include
any communication endpoint. For example, a router also is an end-
system from the point of view of terminating TCP connections for BGP
[10] or ssh [46].
2.1.1. Exploiting Poor Software Quality
The simplest DoS attacks on end-systems exploit poor software quality
on the end-systems themselves, and cause that software to simply
crash. For example, buffer-overflow attacks might be used to
compromise the end-system, but even if the buffer-overflow cannot be
used to gain access, it will usually be possible to overwrite memory
and cause the software to crash. Such vulnerabilities can in
principle affect any software that uses data supplied from the
network. Thus, not only might a web server be potentially
vulnerable, but it might also be possible to crash the back-end
software (such as a database) to which a web server provides data.
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Software crashes due to poor coding affect not only application
software, but also the operating system kernel itself. A classic
example is the so-called "ping of death", which became widely known
in 1996 [21]. This exploit caused many popular operating systems to
crash when sent a single fragmented ICMP echo request packet whose
fragments totaled more than the 65535 bytes allowed in an IPv4
packet.
While DoS attacks such as the ping of death are a significant
problem, they are not a significant architectural problem. Once such
an attack is discovered, the relevant code can easily be patched, and
the problem goes away. We should note though that as more and more
software becomes embedded, it is important not to lose the
possibility of upgrading the software in such systems.
2.1.2. Application Resource Exhaustion
Network applications exist in a context that has finite resources.
In processing network traffic, such an application uses these
resources to do its intended task. However, an attacker may be able
to prevent the application from performing its intended task by
causing the application to exhaust the finite supply of a specific
resource.
The obvious resources that might be exhausted include:
o Available memory.
o The CPU cycles available.
o The disk space available to the application.
o The number of processes or threads or both that the application is
permitted to use.
o The configured maximum number of simultaneous connections the
application is permitted.
This list is clearly not exhaustive, but it illustrates a number of
points.
Some resources are self-renewing: CPU cycles fall in this category --
if the attack ceases, more CPU cycles become available.
Some resources such as disk space require an explicit action to free
up -- if the application cannot do this automatically then the
effects of the attack may be persistent after the attack has ceased.
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This problem has been understood for many years, and it is common
practice for logs and incoming email to be stored in a separate disk
partition (/var on Unix systems) in order to limit the impact of
exhaustion.
Some resources are constrained by configuration: the maximum number
of processes and the maximum number of simultaneous connections are
not normally hard limits, but rather are configured limits. The
purpose of such limits is clearly to allow the machine to perform
other tasks in the event the application misbehaves. However, great
care needs to be taken to choose such limits appropriately. For
example, if a machine's sole task is to be an FTP server, then
setting the maximum number of simultaneous connections to be
significantly less than the machine can service makes the attacker's
job easier. But setting the limit too high may permit the attacker
to cause the machine to crash (due to poor OS design in handling
resource exhaustion) or permit livelock (see below), which are
generally even less desirable failure modes.
2.1.3. Operating System Resource Exhaustion
Conceptually, OS resource exhaustion and application resource
exhaustion are very similar. However, in the case of application
resource exhaustion, the operating system may be able to protect
other tasks from being affected by the DoS attack. In the case of
the operating system itself running out of resources, the problem may
be more catastrophic.
Perhaps the best-known DoS attack on an operating system is the TCP
SYN-flood [19], which is essentially a memory-exhaustion attack. The
attacker sends a flood of TCP SYN packets to the victim, requesting
connection setup, but then does not complete the connection setup.
The victim instantiates state to handle the incoming connections. If
the attacker can instantiate state faster than the victim times it
out, then the victim will run out of memory that it can use to hold
TCP state, and so it cannot service legitimate TCP connection setup
attempts. This issue was exacerbated in some implementations by the
use of a small dedicated storage space for half-open connections,
which made the attack easier than it might otherwise have been. In
the case of a poorly coded operating system, running out of resources
may also cause a system crash.
An alternative TCP DoS attack is the Ack-flood [23], which is
essentially a CPU exhaustion attack on the victim. The attacker
floods the victim with TCP packets pretending to be from connections
that have never been established. A busy server that has a large
number of outstanding connections needs to check which connection the
packet corresponds to. Some TCP implementations implemented this
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search rather inefficiently, and so the attacker could use all the
victim's CPU resources servicing these packets rather than servicing
legitimate requests.
We note that strong authentication mechanisms do not necessarily
mitigate against such CPU exhaustion attacks. In fact, poorly
designed authentication mechanisms using cryptographic methods can
exacerbate the problem. If such an authentication mechanism allows
an attacker to present a packet to the victim that requires
relatively expensive cryptographic authentication before the packet
can be discarded, then this makes the attacker's CPU exhaustion
attack easier.
CPU exhaustion attacks can be also be exacerbated by poor OS handling
of incoming network traffic. In the absence of malicious traffic, an
ideal OS should behave as follows:
o As incoming traffic increases, the useful work done by the OS
should increase until some resource (such as the CPU) is saturated.
o From this point on, as incoming traffic continues to increase the
useful work done should be constant.
However, this is often not the case. Many systems suffer from
livelock [33] where, after saturation, increasing the load causes a
decrease in the useful work done. One cause of this is that the
system spends an increasing amount of time processing network
interrupts for packets that will never be processed, and hence a
decreasing amount of time is available for the application for which
these packets were intended.
2.1.4. Triggered Lockouts and Quota Exhaustion
Many user-authentication mechanisms attempt to protect against
password guessing attacks by locking the user out after a small
number of failed authentications. If an attacker can guess or
discover a user's ID, they may be able to trigger such a mechanism,
locking out the legitimate user.
Another way to deny service using protection mechanisms is to cause a
quota to be exhausted. This is perhaps most common in the case of
small web servers being commercially hosted, where the server has a
contract with the hosting company allowing a fixed amount of traffic
per day. An attacker may be able to rapidly exhaust this quota, and
cause service to be suspended. Similar attacks may be possible
against other forms of quota.
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In the absence of such quotas, if the victim is charged for their
network traffic, a financial denial-of-service may be possible.
2.2. DoS Attacks on Routers
Many of the denial-of-service attacks that can be launched against
end-systems can also be launched against the control processor of an
IP router, for example, by flooding the command and control access
ports. In the case of a router, these attacks may cause the router
to stall, or may cause the router to cease processing routing
packets. Even if the router does not stop servicing routing packets,
it may become sufficiently slow that routing protocols time out. In
any of these circumstances, the consequence of routing failure is not
only that the router ceases to forward traffic, but also that it
causes routing protocol churn that may have further side effects.
An example of such a side effect is caused by BGP route flap damping
[11], which is intended to reduce global routing churn. If an
attacker can cause BGP routing churn, route flap damping may then
cause the flapping routes to be suppressed [31]. This suppression
likely causes the networks served by those routes to become
unreachable.
A DoS attack on the router control processor might also prevent the
router from being managed effectively. This may prevent actions
being taken that would mitigate the DoS attack, and it might prevent
diagnosis of the cause of the problem.
2.2.1. Attacks on Routers through Routing Protocols
In addition to their roles as end-systems, most routers run dynamic
routing protocols. The routing protocols themselves can be used to
stage a DoS attack on a router or a network of routers. This
requires the ability to send traffic from addresses that might
plausibly have generated the relevant routing messages, which is
somewhat difficult with interior routing protocols but fairly easy
with External Border Gateway Protocol (eBGP), for instance.
The simplest attack on a network of routers is to overload the
routing table with sufficiently many routes that the router runs out
of memory, or the router has insufficient CPU power to process the
routes [26]. We note that depending on the distribution and
capacities of various routers around the network, such an attack
might not overwhelm routers near to the attacking router, but might
cause problems to show up elsewhere in the network.
Some routing protocol implementations allow limits to be configured
on the maximum number of routes to be heard from a neighbor [27].
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However, limits often make the problem worse rather than better, by
making it possible for the attacker to push out legitimate routes
with spoofed routes, thus creating an easy form of DoS attack.
An alternative attack is to overload the routers on the network by
creating sufficient routing table churn that routers are unable to
process the changes. Many routing protocols allow damping factors to
be configured to avoid just such a problem. However, as with table
size, such a threshold applied inconsistently may allow the spoofed
routes to merge with legitimate routes before the mechanism is
applied, causing legitimate routes to be damped.
The simplest routing attack on a specific destination is for an
attacker to announce a spoofed desirable route to that destination.
Such a route might be desirable because it has low metric, or because
it is a more specific route than the legitimate route. In any event,
if the route is believed, it will cause traffic for the victim to be
drawn towards the attacking router, where it will typically be
discarded.
A more subtle denial-of-service attack might be launched against a
network rather than against a destination. Under some circumstances,
the propagation of inconsistent routing information can cause traffic
to loop. If an attacker can cause this to happen on a busy path, the
looping traffic might cause significant congestion, as well as fail
to reach the legitimate destination.
In the past, there have been cases where different generations of
routers interpreted a routing protocol specification differently. In
particular, BGP specifies that in the case of an error, the BGP
peering should be dropped. However, if some of the routers in a
network treat a particular route as valid and other routers treat the
route as invalid, then it may be possible to inject a BGP route at
one point in the Internet and cause peerings to be dropped at many
other places in the Internet. Unlike many of the examples above,
while such an issue might be a serious short-term problem, this is
not a fundamental architectural problem. Once the problem is
understood, deploying patched routing code can permanently solve the
issue.
2.2.2. IP Multicast-based DoS Attacks
There are essentially two forms of IP multicast: traditional Any-
Source Multicast (ASM), as specified in RFC 1112 [4] where multiple
sources can send to the same multicast group, and Source-Specific
Multicast (SSM) where the receiver must specify both the IP source
address and the group address. The two forms of multicast provide
rather different DoS possibilities.
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ASM protocols such as PIM-SM [6], MSDP [32], and DVMRP [12] typically
cause some routers to instantiate routing state at the time a packet
is sent to a multicast group. They do this to ensure that the
traffic goes to the group receivers and not to non-receivers. Such
protocols are particularly vulnerable to DoS attacks, as an attacker
that sends to many multicast groups may cause both multicast routing
table explosion (and hence control processor memory exhaustion) and
multicast forwarding table exhaustion (and hence forwarding card
memory exhaustion or thrashing).
ASM also permits an attacker to send traffic to the same group as
legitimate traffic, potentially causing network congestion and
denying service to the legitimate group.
SSM does not permit senders to send to arbitrary groups unless a
receiver has requested the traffic. Thus, sender-based attacks on
multicast routing state are not possible with SSM. However, as with
ASM, a receiver can still join a large number of multicast groups
causing routers to hold a large amount of multicast routing state,
potentially causing memory exhaustion and hence denial-of-service to
legitimate traffic.
With IPv6, hosts are required to send ICMP Packet Too Big or
Parameter Problem messages under certain circumstances, even if the
destination address is a multicast address. If the attacker can
place himself in the appropriate position in the multicast tree, a
packet with an unknown but mandatory Destination Option, for
instance, could generate a very large number of responses to the
claimed sender.
With IPv4, the same problem exists with multicast ICMP Echo Request
packets, but these are somewhat easier to filter.
The examples above should not be taken as exhaustive. These are
actually specific cases of a general problem that can happen when a
multicast/broadcast request solicits a reply from a large number of
nodes.
2.2.3. Attacks on Router Forwarding Engines
Router vendors implement many different mechanisms for packet
forwarding, but broadly speaking they fall into two categories: ones
that use a forwarding cache, and ones that do not. With a forwarding
cache, the forwarding engine does not hold the full routing table,
but rather holds just the currently active subset of the forwarding
table.
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Many modern routers use a loosely coupled architecture, where one or
more control processors handle the routing protocols and communicate
over an internal network link to special-purpose forwarding engines,
which actually forward the data traffic. In such architectures, it
may be possible for an attacker to overwhelm the communications link
between the control processor and the forwarding engine. This is
possible because the forwarding engines support very high speed
links, and the control processor simply cannot handle a similar rate
of traffic.
There may be many ways in which an attacker can trigger communication
between the forwarding engines and the control processor. The
simplest way is for the attacker to simply send to the router's IP
address, but this should in principle be relatively easy to prevent
using filtering on the forwarding engines. Another way might be to
cause the router to forward data packets using the "slow path". This
involves sending packets that require special attention from the
forwarding router; if the forwarding engine is not smart enough to
perform such forwarding, then it will typically pass the packet to
the control processor. In a router using a forwarding cache, it may
be possible to overload the internal communications by thrashing the
forwarding cache. Finally, any form of data-triggered communication
between the forwarding engine and the control processor might cause
such a problem. Certain multicast routing protocols including PIM-SM
contain many such data triggered events that could potentially be
problematic.
The effects of overloading such internal communications are hard to
predict and are very implementation-dependent. One possible effect
might be that the forwarding table in the forwarding engine gets out
of synchronization with the routing table in the control processor
that reflects what the routing protocols believe is happening. This
might cause traffic to be dropped or to loop.
Finally, if an attacker can generate traffic that causes a router to
auto-install access control list (ACL) entries, perhaps by triggering
a response from an intrusion detection system, then it may be
possible to exhaust the ACL resources on the router. This might
prevent future attacks from being filtered, or worse, cause ACL
processing to be handled by the route processor.
2.3. Attacks on Ongoing Communications
Instead of attacking the end-system itself, it is also possible for
an attacker to disrupt ongoing communications. If an attacker can
observe a TCP connection, then it is relatively easy for them to
spoof packets to either reset that connection or to de-synchronize it
so that no further progress can be made [29]. Such attacks are not
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prevented by transport or application-level security mechanisms such
as TLS [5] or ssh, because the authentication takes place after TCP
has finished processing the packets.
If an attacker cannot observe a TCP connection, but can infer that
such a connection exists, it is theoretically possible to reset or
de-synchronize that connection by spoofing packets into the
connection. However, this might require an excessively large number
of spoofed packets to guess both the port of the active end of the
TCP connection (in most cases, the port of the passive end is
predictable) and the currently valid TCP sequence numbers. However,
as some operating systems have poorly implemented predictable
algorithms for selecting either the dynamically selected port or the
TCP initial sequence number [41] [20], then such attacks have been
found to be feasible [34]. Advice as to how to reduce the
vulnerability in the specific case of TCP is available in [37].
An attacker might be able to significantly reduce the throughput of a
connection by sending spoofed ICMP source quench packets, although
most modern operating systems should ignore such packets. However,
care should be taken in the design of future transport and signaling
protocols to avoid the introduction of similar mechanisms that could
be exploited.
2.4. Attacks Using the Victim's Own Resources
Instead of directly overloading the victim, it may be possible to
cause the victim or a machine on the same subnet as the victim to
overload itself.
An example of such an attack is documented in [18], where the
attacker spoofs the source address on a packet sent to the victim's
UDP echo port. The source address is that of another machine that is
running a UDP chargen server (a chargen server sends a character
pattern back to the originating source). The result is that the two
machines bounce packets back and forth as fast as they can,
overloading either the network between them or one of the end-systems
itself.
2.5. DoS Attacks on Local Hosts or Infrastructure
There are a number of attacks that might only be performed by a local
attacker.
An attacker with access to a subnet may be able to prevent other
local hosts from accessing the network at all by simply exhausting
the address pool allocated by a Dynamic Host Configuration Protocol
(DHCP) server. This requires being able to spoof the MAC address of
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an ethernet or wireless card, but this is quite feasible with certain
hardware and operating systems.
An alternative DHCP-based attack is simply to respond faster than the
legitimate DHCP server, and to give out an address that is not useful
to the victim.
These sorts of bootstrapping attacks tend to be difficult to avoid
because most of the time trust relationships are established after IP
communication has already been established.
Similar attacks are possible through ARP spoofing [16]; an attacker
can respond to ARP requests before the victim and prevent traffic
from reaching the victim. Some brands of ethernet switch allow an
even simpler attack: simply send from the victim's MAC address, and
the switch will redirect traffic destined for the victim to the
attacker's port. This attack might also potentially be used to block
traffic from the victim by engaging screening or flap-dampening
algorithms in the switch, depending on the switch design.
It may be possible to cause broadcast storms [16] on a local LAN by
sending a stream of unicast IP packets to the broadcast MAC address.
Some hosts on the LAN may then attempt to forward the packets to the
correct MAC address, greatly amplifying the traffic on the LAN.
802.11 wireless networks provide many opportunities to deny service
to other users. In some cases, the lack of defenses against DoS was
a deliberate choice--because 802.11 operates on unlicensed spectrum
it was assumed that there would be sources of interference and that
producing intentional radio-level jamming would be trivial. Thus,
the amount of DoS protection possible at higher levels was minimal.
Nevertheless, some of the weaknesses of the protocols against more
sophisticated attacks are worth noting. The most prominent of these
is that association is unprotected, thus allowing rogue access points
(APs) to solicit notifications that would otherwise have gone to
legitimate APs.
The SSID field provides effectively no defense against this kind of
attack. Unless encryption is enabled, it is trivial to announce the
presence of a base station (or even of an ad-hoc mode host) with the
same network name (SSID) as the legitimate basestation. Even adding
authentication and encryption a la 802.1X and 802.11i may not help
much in this respect. The SSID space is unmanaged, so everyone is
free to put anything they want in the SSID field. Most host stacks
don't deal gracefully with this. Moreover, SSIDs are very often set
to the manufacturer's default, making them highly predictable.
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Some 802.11 basestations have limited memory for the number of
associations they can support. If this is exceeded, they may drop
all associations. In an attempt to forestall this problem, some APs
advertise their load so as to enable stations to choose APs that are
less loaded. However, crude implementations of these algorithms can
result in instability.
Finally, as the authentication in 802.11 takes place at a
comparatively high level in the stack, it is possible to simply
deauthenticate or disassociate the victim from the basestation, even
if Wired Equivalent Privacy (WEP) is in use [30]. Bellardo and
Savage [15] describe some simple remedies that reduce the
effectiveness of such attacks. While IEEE 802.11w will protect
Deauthenticate or Disassociate frames, this attack is still possible
via forging of Association frames.
What all these attacks have in common is that they exploit
vulnerabilities in the link auto-configuration mechanisms. In a
wireless network, it is necessary for a station to detect the
presence of APs in order to choose which one to connect to. In
802.11, this is handled via the Beacon and Probe Request/Response
mechanisms.
Beacons cannot easily be encrypted, because the station needs to
utilize them prior to authentication in order to discover which APs
it may wish to communicate with. Since authentication can only occur
after interpreting the Beacon, an encrypted Beacon would present a
chicken-egg problem: you can't obtain a key to decrypt the Beacon
until completing authentication, and you may not be able to figure
out which AP to authenticate with prior to decrypting the Beacon.
Note that in principle you could encrypt Beacons with a shared
(per-AP) key but this would require each station to trial-decrypt
beacons until it finds one that matches up to whatever shared
authentication secret it had. This is not particularly convenient.
As a result, discussions of Beacon frame security have largely
focused on authentication of Beacon frames, not encryption. Even
here, solutions are difficult. While it may be possible for a
station to validate a Beacon *after* authentication (either by
checking a Message Integrity Check (MIC) computed with the group key
provided by the AP or verifying the Beacon parameters during the
4-way handshake), doing so *before* authentication may require
synchronization of keys between APs within an SSID.
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RFC 4732 DoS Considerations November 20062.6. DoS Attacks on Sites through DNS
In today's Internet, DNS is of sufficient importance that if access
to a site's DNS servers is denied, the site is effectively
unreachable, even if there is no actual communication problem with
the site itself.
Many of the attacks on end-systems described above can be perpetrated
on DNS servers. As servers go, DNS servers are not particularly
vulnerable to DoS. So long as a DNS server has sufficient memory, a
modern host can usually respond very rapidly to DNS requests for
which it is authoritative. This was demonstrated in October 2002
when the root nameservers were subjected to a very large DoS attack
[38]. A number of the root nameservers have since been replicated
using anycast [1] to further improve their resistance to DoS.
However, it is important for authoritative servers to have relaying
disabled, or it is possible for an attacker to force the DNS servers
to hold state [40].
Many of the routing attacks can also be used against DNS servers by
targeting the routing for the server. If the DNS server is co-
located with the site for which is authoritative, then the fact that
the DNS server is also unavailable is of secondary importance.
However, if all the DNS servers are made unavailable, this may cause
email to that site to bounce rather than being stored while the mail
servers are unreachable, so distribution of DNS server locations is
important.
Causing network congestion on links to and from a DNS server can have
similar effects to end-system attacks or routing attacks, causing DNS
to fail to obtain an answer, and effectively denying access to the
site being served.
We note that if an attacker can deny external access to all the DNS
servers for a site, this will not only cause email to that site to be
dropped, but it will also cause email from that site to be dropped.
This is because recent versions of mail transfer agents such as
sendmail will drop email if the mail originates from a domain that
does not exist. This is a classic example of unexpected
consequences. Sendmail performs this check as an anti-spam measure,
and spam itself can be viewed as a form of DoS attack. Thus,
defending against one DoS attack opens up the vulnerability that
allows another DoS attack. If a receiving implementation is using a
black-hole list (see Section 2.14) served by DNS, an attacker can
also mount a DoS attack by attacking the black-hole server.
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Finally, a data corruption attack is possible if a site's nameserver
is permitted to relay requests from untrusted third parties [40].
The attacker issues a query for the data he wishes to corrupt, and
the victim's nameserver relays the request to the authoritative
nameserver. The request contains a 16-bit ID that is used to match
up the response with the request. If the attacker spoofs sufficient
response packets from the authoritative nameserver just before the
official response arrives, each containing a forged response and a
different DNS ID, then there is a reasonable chance that one of the
forged responses will have the correct DNS ID. The incorrect data
will then be believed and cached by the victim's nameserver, so
giving the incorrect response to future queries. The probability of
the attack can further be increased if the attacker issues many
different requests for the same data with different DNS IDs, because
many nameserver implementations will issue relayed requests with
different DNS IDs, and so the response only has to match any one of
these request IDs [17] [36].
The use of anycast for DNS services makes it even more vulnerable to
spoofing attacks. An attacker who can convince the ISP to accept an
anycast route to his fake DNS server can arrange to receive requests
and generate fake responses. Anycast DNS also makes DoS attacks on
DNS easier. The idea is to disable one of the DNS servers while
maintaining the BGP route to that server. This creates failures for
any client that is routed to the (now defunct) server.
2.7. DoS Attacks on Links
The simplest DoS attack is to simply send enough non-congestion-
controlled traffic such that a link becomes excessively congested,
and legitimate traffic suffers unacceptably high packet loss.
Under some circumstances, the effect of such a link DoS can be much
more extensive. We have already discussed the effects of denying
access to a DNS server. Congesting a link might also cause a routing
protocol to drop an adjacency if sufficient routing packets are lost,
potentially greatly amplifying the effects of the attack. Good
router implementations will prioritize the transmission of routing
packets, but this is not a total panacea. If routers are peered
across a shared medium such as ethernet, it may be possible to
congest the medium sufficiently that routing packets are still lost.
Even if a link DoS does not cause routing packets to be lost, it may
prevent remote access to a router using ssh or Simple Network
Management Protocol (SNMP) [48]. This might make the router
unmanageable, or prevent the attack from being correctly diagnosed.
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The prioritization of routing packets can itself cause a DoS problem.
If the attacker can cause a large amount of routing flux, it may be
possible for a router to send routing packets at a high enough rate
that normal traffic is effectively excluded. However, this is
unlikely except on low-bandwidth links.
Finally, it may be possible for an attacker to deny access to a link
by causing the router to generate sufficient monitoring or report
traffic that the link is filled. SNMP traps are one possible vector
for such an attack, as they are not normally congestion controlled.
Attackers with physical access to multiple access links can easily
bring down the link. This is particularly easy to mount and
difficult to counter with wireless networks.
2.8. DoS Attacks on Firewalls
Firewalls are intended to defend the systems behind them against
attack. In that they restrict the traffic that can reach those
systems, they may also aid in defending against denial-of-service
attacks. However, under some circumstances the firewall itself may
also be used as a weapon in a DoS attack.
There are many different types of firewall, but generally speaking
they fall into stateful and stateless classes. The state here refers
to whether the firewall holds state for the active flows traversing
the firewall. Stateless firewalls generally can only be attacked by
attempting to exhaust the processing resources of the firewall.
Stateful firewalls can be attacked by sending traffic that causes the
firewall to hold excessive state or state that has pathological
structure.
In the case of excessive state, the firewall simply runs out of
memory, and can no longer instantiate the state required to pass
legitimate flows. Most firewalls will then fail disconnected,
causing denial-of-service to the systems behind the firewall.
In the case of pathological structure, the attacker sends traffic
that causes the firewall's data structures to exhibit worst-case
behaviour. An example of this would be when the firewall uses hash
tables to look up forwarding state, and the attacker can predict the
hash function used. The attacker may then be able to cause a large
amount of flow state to hash to the same bucket, which causes the
firewall's lookup performance to change from O(1) to O(n), where n is
the number of flows the attacker can instantiate [28]. Thus, the
attacker can cause forwarding performance to degrade to the point
where service is effectively denied to the legitimate traffic
traversing the firewall.
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RFC 4732 DoS Considerations November 20062.9. DoS Attacks on IDS Systems
Intrusion detection systems (IDSs) suffer from similar problems to
firewalls. It may be possible for an attacker to cause the IDS to
exhaust its available processing power, to run out of memory, or to
instantiate state with pathological structure. Unlike a firewall, an
IDS will normally fail open, which will not deny service to the
systems protected by the IDS. However, it may mean that subsequent
attacks that the IDS would have detected will be missed.
Some IDSs are reactive; that is, on detection of a hostile event they
react to block subsequent traffic from the hostile system, or to
terminate an ongoing connection from that system. It may be possible
for an attacker to spoof packets from a legitimate system, and hence
cause the IDS to believe that system is hostile. The IDS will then
cause traffic from the legitimate system to be blocked, hence denying
service to it. The effect can be particularly bad if the legitimate
system is a router, DNS server, or other system whose performance is
essential for the operation of a large number of other systems.
2.10. DoS Attacks on or via NTP
Network time servers are generally not considered security-critical
services, but under some circumstances NTP servers might be used to
perpetrate a DoS attack.
The most obvious such attack is to DoS the NTP servers themselves.
Many end-systems have rather poor clock accuracy and so, without
access to network time, their clock will naturally drift. This can
cause problems with distributed systems that rely on good clocks.
For example, one commonly used revision control system can fail if it
perceives the modification timestamp to be in the future.
If the NTP servers relied on by a host can be subverted, either
through compromising or impersonating them, then the attacker may be
able to control the host's system clock. This can cause many
unexpected consequences, including the premature expiry of dated
resources such as encryption or authentication keys. This in turn
can prevent access to other more critical services.
2.11. Physical DoS
The discussion thus far has centered on denial-of-service attacks
perpetrated using the network. However, computer systems are only as
resilient as the weakest link. It may be easier to deny service by
causing a power failure, by cutting network cables, or by simply
switching a system off, and so physical security is at least as
important as network security. Physical attacks can also serve as
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entry points for non-physical DoS, for instance, by reducing the
resources available to deal with overcapacity.
2.12. Social Engineering DoS
The weakest link may also be human. In defending against DoS, the
possibility of denial-of-service through social engineering should
not be neglected, such as convincing an employee to make a
configuration change that prevents normal operation.
2.13. Legal DoS
Computer systems cannot be considered in isolation from the social
and legal systems in which they operate. This document focuses
primarily on the technical issues, but we note that "cease and
desist" letters, government censorship, and other legal mechanisms
also touch on denial-of-service issues.
2.14. Spam and Black-Hole Lists
Unsolicited commercial email, also known as "spam", can effectively
cause denial-of-service to email systems. While the intent is not
denial-of-service, the large amount of unwanted mail can waste the
recipient's time or cause legitimate email to fail to be noticed
amongst all the background noise. If spam filtering software is
used, some level of false positives is to be expected, and so these
messages are effectively denied service.
One mechanism to reduce spam is the use of black-hole lists. The IP
addresses of dial-up ISPs or mail servers used to originate or relay
spam are added to black-hole lists. The recipients of mail choose to
consult these lists and reject spam if it originates or is relayed by
systems on the list. One significant problem with such lists is that
it may be possible for an attacker to cause a victim to be black-
hole-listed, even if the victim was not responsible for relaying
spam. Thus, the black-hole list itself can be a mechanism for
effecting a DoS attack. Note that every black-hole list has its own
policy regarding additions, and some are less susceptible to this DoS
attack than others. Consumers of black-hole list technology are
advised to investigate these policies before they subscribe. Similar
considerations apply to feeds of bad BGP bad route advertisements.
Handley, et al. Informational [Page 19]

RFC 4732 DoS Considerations November 20063. Attack Amplifiers
Many of the attacks described above rely on sending sufficient
traffic to overwhelm the victim. Such attacks are made much easier
by the existence of "attack amplifiers", where an attacker can send
traffic from the spoofed source address of the victim and cause
larger responses to be returned to the victim. A detailed discussion
of such reflection attacks can be found in [35].
3.1. Methods of Attack Amplification
The simplest such attack was the "smurf" attack [22], where an ICMP
echo request packet with the spoofed source address of the victim is
sent to the subnet-broadcast address of a network to be used as an
amplifier. Every system on that subnet then responds with an ICMP
echo response that returns to the victim. Smurf attacks are no
longer such a serious problem, as these days routers usually drop
such packets and end-systems do not respond to them.
An alternative form of attack amplifier is typified by a DNS
reflection attack. An attacker sends a DNS request to a DNS server
requesting resolution of a domain name. Again the source address of
the request is the spoofed address of the victim. The request is
carefully chosen so that the size of the response is significantly
greater than the size of the request, thereby providing the
amplification. As an aside, it is interesting to note that the
largest DNS responses tend to be those incorporating DNSsec
authentication information. This attack amplifier can only be used
by an attacker with the ability to spoof the source address of the
victim. However, we note that if the victim's DNS server is
configured to relay requests from external clients, it may be
possible to cause it to congest its own incoming network link.
Another variant of attack amplifier involves amplification through
retransmission. This is typified by a TCP amplification attack known
as "bang.c". The attacker sends a spoofed TCP SYN with the source
address of the victim to an arbitrary TCP server. The server will
respond with a SYN|ACK that is sent to the victim, and when no final
ACK is received to complete the handshake, the SYN|ACK will be
retransmitted a number of times. Typically, this attack uses a very
large list of arbitrarily chosen servers as reflectors. For the
attack to be successful, the reflector must not receive a RST from
the victim in response to the SYN|ACK. However, if the attack
traffic sufficiently overwhelms the server or access link to the
server, then packet loss will ensure that many reflectors do not
receive a RST in response to their SYN|ACK, and so continue to
retransmit. The attack can be exacerbated by firewalls that silently
drop the incoming SYN|ACK without sending a RST.
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Care must also be taken with services that relay requests. If an
attacker can send a request to a proxy, and that proxy now attempts
to connect to a victim whose address is chosen by the attacker, then,
if the proxy repeatedly resends the request when receiving no answer,
this can also serve as an attack amplifier.
Another variant of amplification occurs in protocols that include,
within the protocol payload, an IP address or name of host to which
subsequent messages should be sent. An example of such a protocol is
the Session Initiation Protocol (SIP) [50], which carries a payload
defined by the Session Description Protocol (SDP) [51]. The SDP
payload of the SIP message conveys the IP address and port to which
media packets, typically encoded using the Real Time Transport
Protocol (RTP) [52], are sent.
To launch this attack, an attacker sends a protocol message, and sets
the IP address within the payload to point to the attack target. The
recipient of the message will generate subsequent traffic to that IP
address. Depending on the protocol, this attack can provide
substantial amplification properties. In the specific case of SIP,
if a caller makes calls to high-bandwidth media sources (such as a
video server or streaming audio server), a single SIP INVITE packet,
typically a few hundred bytes, can result in a nearly continuous
stream of media packets at rates anywhere from a few kbits per second
up to megabits per second. This particular attack is called the
"voice hammer".
Unlike the other techniques described above, this technique does not
require the attacker to modify packets or even spoof their source IP
address. This makes it easier to launch.
This attack is prevented through careful protocol design. Protocols
should, whenever possible, avoid including IP addresses or hostnames
within protocol payloads as addresses to which subsequent messaging
should be sent. Rather, when possible, messages should be sent to
the source IP from which the protocol packet came. If such a design
is not possible, the protocol should include a handshake whereby it
can be positively determined that the protocol entity at that IP
address or hostname does, in fact, wish to receive that subsequent
messaging. That handshake itself needs to be lightweight (to avoid
being the source of another DoS attack), and secured against the
spoofing of the handshake response.
Finally, a somewhat similar attack is possible with some protocols
where one message leads to another message that is not sent as a
reply to the source address of the first message. This can be an
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issue with protocols to enable mobility, for example, and might
permit an attacker to avoid ingress filtering. Such protocols are
notoriously difficult to get right.
3.2. Strategies to Mitigate Attack Amplification
In general, the architectural lessons to be learnt are simple:
o As far as possible, perform ingress filtering [7] [39] to prevent
source address spoofing.
o Avoid designing protocols or mechanisms that can return
significantly larger responses than the size of the request,
unless a handshake is performed to validate the client's source
address. Such a handshake needs to incorporate an unpredictable
nonce that is secure enough to mitigate the amplification effects
of the protocol.
o All retransmission during initial connection setup should be
performed by the client.
o Proxies should not arbitrarily relay requests to destinations
chosen by a client.
o Avoid signaling third-party connections. Any unavoidable third-
party connections set up by a signaling protocol should
incorporate lightweight validation before sending significant
data.
4. DoS Mitigation Strategies
A general problem with DoS defense is that it is not in principle
possible to distinguish between a flash crowd and a DoS attack.
Indeed, having your site taken down by a flash crowd is probably a
more common experience than having it DoS-ed -- so common it has
acquired its own names: being Slashdotted or Farked, after the web
sites that are common sources of flash crowds. Thus, the first line
of defense against DoS attacks must be to provision your service so
that it can handle a foreseeable legitimate peak load.
Underprovisioned sites are the easiest to take down.
Specific strategies for DoS defense fall into two broad categories:
1. Avoiding allowing attacks that are better than generic resource
consumption.
2. Minimizing the extent to which generic resource consumption
attacks crowd out legitimate users.
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In the remainder of this section, we consider specific applications
of these two approaches at a variety of levels of network system
architecture.
4.1. Protocol Design4.1.1. Don't Hold State for Unverified Hosts
From an end-system server point of view, one simple aim is to avoid
instantiating state without having completed a handshake with the
client to validate their address, and as far as possible to push work
and stateholding to client. There are a number of techniques that
might be used to do this, including SYN cookies [2] [14]. All
client-server protocols should probably be designed to allow such
techniques to be used, but the enabling of the mechanism should
normally be at the server's discretion to avoid unnecessary work
under normal circumstances.
4.1.2. Make It Hard to Simulate a Legitimate User
Other than having massive overcapacity, the only real defense against
resource consumption attacks is to preferentially discriminate
against attackers. The general idea is to find something that
legitimate users can do but attackers can't. The most commonly
proposed approaches include:
1. Puzzles: force the attacker to do some computation that would not
be onerous for a single user but is too expensive to do en masse
[14].
2. Reverse Turing tests: specialized puzzles that are hard for
machines to do but easy for humans, thus making automated attacks
hard [13].
3. Reachability testing: force the proposed client to demonstrate
that it can receive traffic at a given IP address. This makes it
easier to trace attackers.
All of these techniques have substantial limitations. Puzzles tend
to discriminate against legitimate users with slow computers. In
addition, the wide availability of remotely controlled compromised
machines ("bots") means that attackers have ample computing power at
their disposal. There has been substantial work in attacking reverse
Turing tests automatically, thus making them of limited
applicability. Finally, reachability testing is substantially
weakened by bots because the attacker does not need to hide his
source address.
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RFC 4732 DoS Considerations November 20064.1.3. Graceful Routing Degradation
A goal with routing protocols is that of graceful degradation in
overload, and automatic recovery after the source of the overload has
been remedied. Some routing protocols satisfy this goal more than
others. Although RIP [53] doesn't scale well, if a router runs out
of memory when receiving a RIP route, it can just drop the route and
send an infinite metric to its peers. The route will later be
refreshed, and if the original source of the problem has been
resolved, the router will now be able to process it correctly.
On the other hand, BGP is stateful in the sense that a peer assumes
you have processed or chosen to filter any route that it sent you.
There is no mechanism to refresh state in the base BGP spec, and even
the later route refresh option [3] is hard to use in the presence of
overload. A BGP router that cannot store a route it received has two
choices: completely restart BGP or shut down one or more peerings
[26]. This means that the effects of a BGP overload are rather more
severe than they need to be, and so amplifies the effect of any
attack.
In general, few routing protocol designs actively consider the
possible behaviour of routers under overload conditions; this should
be an explicit part of future routing protocol designs. Although
precise details should clearly be left to implementors, the protocol
design needs to give them the capability to do their job properly.
4.1.4. Autoconfiguration and Authentication
Autoconfiguration mechanisms greatly ease deployment, and are
increasingly necessary as the number of networked devices grows
beyond what can be managed manually. However, it should be
recognised that unauthenticated autoconfiguration opens up many
avenues for attack. There is a clear tension between ease of
configuration and security of configuration, especially because there
are environments in which it is desirable for units to operate with
effectively no authentication (e.g., airport hotspots). Future
autoconfiguration protocols should consider the need to allow
different end-systems to operate at different points in this spectrum
within the same autoconfiguration framework. However, this also
implies that the network elements should avoid acting for
unauthenticated hosts, instead just letting them access the network
more or less directly.
Handley, et al. Informational [Page 24]

RFC 4732 DoS Considerations November 20064.2. Network Design and Configuration
In general, networks should be provisioned with private, out-of-band
access to console or control ports so that such control facilities
will be available in the face of a DoS attack launched against either
the control or data plane of the (in-band) network. Typically, such
out-of-band networks are provisioned on a separate infrastructure for
exactly this purpose. Out-of-band access is a crucial capability for
DoS mitigation, since many of the typical redundancy and capacity
management techniques (such as prioritizing routing or network
management traffic) fail during such attacks. In addition, many
redundancy protocols such as VRRP [47] can fail during such attacks
as they may be unable to keep adjacencies alive.
There are several default configuration settings that can also be
exploited to generate several of the attacks outlined in this
document. For example, some vendors may have features such as IP
redirect, directed broadcast, and proxy ARP enabled by default.
Similar defaults, such as publicly readable SNMP [48] communities
(e.g., "public") can be used to reveal otherwise confidential
information to a prospective attacker. Finally, other
unauthenticated configuration management protocols such as TFTP [49]
should be avoided if possible; at the very least access to TFTP
configuration archives should be protected and TFTP should be
filtered at administrative boundaries. Finally, since many of the
password encryption techniques used by router vendors are reversible,
keeping such passwords on a configuration archive (as part of a
configuration file), even in the encrypted form written by the
router, can lead to unauthorized access if the archive is
compromised.
4.2.1. Redundancy and Distributed Service
A basic principle of designing systems to handle failure is to have
redundant servers that can take over when one fails. This is equally
true in the case of DoS attacks, which often focus on a given server
and/or link. If service delivery points can be distributed across
the network, then it becomes much harder to attack the entire
service. In particular, this makes attacks on a single network link
more difficult.
4.2.2. Authenticate Routing Adjacencies
In general, cryptographic authentication mechanisms are too costly to
form the main part in DoS prevention. However, routing adjacencies
are too important to risk an attacker being able to inject bad
routing information, which can affect more than the router in
question. Additional non-cryptographic mechanisms should then be
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used to avoid arbitrary end-systems being able to cause the router to
spend CPU cycles on validating authentication data.
For BGP, at the very least, this implies the use of TCP MD5 [9] or
IPsec authentication, combined with the GTSM [8] to prevent eBGP
association with non-immediate neighbors. In the future, this will
likely imply better authentication of the routing information itself.
4.2.3. Isolate Router-to-Router Traffic
As far as is feasible, router-to-router traffic should be isolated
from data traffic. How this should be implemented depends on the
precise technologies available, both in the router and at the link
layer. The goal should be that failure of the link for data traffic
should also cause failure for the routing traffic, but that an
attacker cannot directly send packets to the control processor of the
routers.
A downside of this is that some diagnostic techniques (such as
pinging consecutive routers to find the source of a delay) may no
longer be possible. Ideally, alternative mechanisms (which do not
open up additional avenues for DoS) should be designed to replace
such lost techniques.
4.3. Router Implementation Issues
Because a router can be considered as an end-system, it can
potentially benefit from all the prevention mechanisms prescribed for
end-system implementation. However, one basic distinction between a
router and a host is that the former implements routing protocols and
forwards data, which in turn lead to additional router-specific
implementation considerations. The issues described below are meant
to be illustrative and not exhaustive.
4.3.1. Checking Protocol Syntax and Semantics
Protocol syntax defines the formation of the protocol messages and
the rules of exchanges. The questions addressed by protocol syntax
checking includes, but is not limited to, the following:
1. Who sent the message?
2. Does the content conform to the protocol format?
3. Was the message sent with correct timing?
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The first step in protocol syntax verification is to ensure that an
incoming message was sent by a legitimate party. There are multiple
ways to perform this check. One can verify the source IP address and
even the MAC address of the message. Utilizing the fact that eBGP
peers are normally directly connected, one can also check the TTL
value in a packet and discard any BGP updates packet whose TTL is
less than some maximum value (typically, max TTL - 1) [8].
Cryptographic authentication should also be used whenever available
to verify that an incoming message is indeed from an expected sender.
For BGP, at the very least, this implies the use of TCP MD5 [9] or
IPsec authentication.
In addition to the sender verification, it is also important to check
the syntax of a received routing message, as opposed to assuming that
all messages came in a correct format. It happened in the past that
routers crashed upon receiving ill-formed routing messages. Such
faults will be prevented by performing rigorous syntax checking.
4.3.2. Consistency Checks
Protocol semantics define the meaning of the message content, the
interpretation of the values, and the actions to be taken according
to the content. Here is a simple example of using semantic checking.
When a link failure causes a router in Autonomous System (AS) A to
send a peer router B a withdrawal message for prefix P, B should make
sure that any alternative path it finds to reach P does not go
through A. This simple check is shown to significantly improve BGP
convergence time in many cases [42].
Another example of using semantic checking against false routing
injection is described in [44]. The basic idea is to attach to the
route announcement for prefix P a list of the valid origin ASes. Due
to the rich connectivity in today's Internet topology, a remote AS
will receive routing updates from multiple different paths and can
check to see whether each update carries the identical origin AS
list. Although a false origin may announce reachability to P, or
alter the origin AS list, it would be difficult, if not impossible,
to block the correct updates from propagating out, and thus remote
ASes can detect the existence of false updates by observing the
inconsistency of the received origin AS lists for P. Research
studies show that the "allowed origin list" test can effectively
detect the majority of falsely originated updates.
Generally speaking, verifying the validity of BGP routes can be
challenging because BGP is policy driven and policies of individual
ISPs are not known in most cases. But assuming that policies do not
change in short time scale, in principle one could verify new updates
against observed routes from the recent past, which reflect the
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RFC 4732 DoS Considerations November 2006
routing policies in place. Research work is needed to explore this
direction.
Note that while the above steps are all fairly simple and don't
really "bulletproof" the protocol, each adds some degree of
protection. As such, the combination of the above techniques can
result in an effective reduction in the probability of undetected
faults.
4.3.3. Enhance Router Robustness through Operational Adjustments
There exist a number of configuration tunings that can enhance
robustness of BGP operations. One example is to let BGP peers
coordinate the setting of a limit on the number of prefixes that one
BGP speaker will send to its peer [43]. Although such a check does
not validate the prefix owned by each peer, it can prevent false
announcements of large numbers of invalid routes. Had all BGP
routers been configured with this simple checking earlier, several
large-scale routing outages in the past could have been prevented.
Note, however, that care must be taken with hard limits of this type
because they can be used to mount a DoS because implementations often
discard excess routes indiscriminately, thus potentially causing
black-holing of correct routes.
Another example of useful configuration tuning is to adjust the BGP's
KeepAlive and Hold Timer values to minimize BGP peering session
resets. Previous measurements show that heavy traffic load, such as
those caused by worms, can cause BGP KeepAlive messages to be delayed
or dropped, which in turn cause BGP peering session breakdown. Such
load-induced session breaks and re-establishments can lead to an
excessive amount of BGP updates during the periods when stable
routing is needed most.
4.3.4. Proper Handling of Router Resource Exhaustion
In addition to the resource exhaustion problems that are generally
apply to all end-systems, as described in Section 2, router
implementations must also take special care in handling resource
exhaustions when they occur in order to keep the router operating
despite the problem. For example, under normal operations a router
does not require a large cache to hold outstanding ARP requests
because the replies are normally received within a few milliseconds.
However, certain conditions can lead to ARP cache exhaustion, for
example, during a virus attack where many packets are sent to non-
existing IP addresses, thus there are no ARP replies to the requests
for those addresses. Such phenomena have happened in the past and
led to routers failing to forward packets.
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RFC 4732 DoS Considerations November 2006
Another example is queue management. Many high-end routers are
designed so that most packets can be handled purely in specialized
processors (Application-Specific Integrated Circuit (ASICs), Field
Programmable Gate-Arrays (FPGAs), etc.). However, exceptional
packets must be routed to a supporting general purpose CPU for
handling. On some such systems, it may be possible mount a low-
effort DoS attack by saturating the queues between the specialized
hardware and the supporting processor.
So the attack vector on routers/network devices is a low packets-
per-second (PPS) queue saturation attack on the ASIC's raw queue
structure. The countermeasure here is to have multiple such queues
designed in such a way that it's difficult for an attacker to arrange
to fill multiple queues [45].
4.4. End-System Implementation Issues4.4.1. State Lookup Complexity
Any system that instantiates per-connection state should take great
care to implement state-lookup mechanisms in such a way that
performance cannot be controlled by the attacker. One way to achieve
this is to use hash tables where the hash mechanism is keyed in such
a way that the attacker cannot instantiate a large number of flows in
the same hash bucket.
4.4.1.1. Avoid Livelock
Most operating systems use network interrupts to receive data from
the network, which is a good solution if the host spends only a small
amount of its time handling network traffic. However, this leaves
the host open to livelock [33], where under heavy load the OS spends
all its time handling interrupts and no time doing the work needed to
handle the traffic at the application level. Server operating
systems should consider using network polling at times of heavy load,
rather than being interrupt-driven, and should be carefully
architected so that as far as reasonably possible, traffic received
by the OS is processed to completion or very cheaply discarded.
4.4.1.2. Use Unpredictable Values for Session IDs
Most recent TCP implementations use fairly good random mechanisms for
allocating the TCP initial sequence numbers. In general, any
dynamically allocated value used purely to identify a communication
session should be allocated using an unpredictable mechanism, as this
increases the search space for an attacker that wishes to disrupt
ongoing communications. Thus, the dynamically allocated port of the
active end of a TCP connection might also be randomly allocated.
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With DNS, the ID that is used to match responses with requests should
also be randomly generated. However, as the ID field is only 16
bits, the protection is rather limited.
4.4.2. Operational Issues4.4.2.1. Eliminate Bad Traffic Early
Many DoS attacks are generic bandwidth consumption attacks that
operate by clogging the link that connects the victim server to the
Internet. Filtering these attacks at the server does no good because
the traffic has already traversed the link that is the scarce
resource. Such flows need to be filtered at some point closer to the
attacker. Where possible, operators should filter out obviously bad
traffic. In particular, they should perform ingress filtering [7].
4.4.2.2. Establish a Monitoring Framework
Network operators are strongly encouraged to establish a monitoring
framework to detect and log abnormal network activity. One cannot
defend against an attack that one doesn't detect or understand. Such
monitoring tools can be used to set a baseline of "normal" traffic,
and can be used to detect aberrant flows and determine the type and
source of the aberrant flows. This is extremely helpful when
responding to distributed DoS attacks or a flash crowd, and should be
in place prior to the event.
5. Conclusions
In this document, we have highlighted possible avenues for DoS
attacks on networks and networked systems, with the aim of
encouraging protocol designers and network engineers towards designs
that are more robust. We have discussed partial solutions that
reduce the effectiveness of attacks, and highlighted how some partial
solutions can be taken advantage of by attackers to perpetrate
alternative attacks.
Our focus has primarily been on protocol and network architecture
issues, but there are many things that network and service operators
can do to lessen the threat. Further advice and information for
network operators can be found in [24] [39] [25].
It is our hope that this document will spur discussion leading to
architectural solutions that reduce the succeptibility of all
Internet systems to denial-of-service attacks.
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RFC 4732 DoS Considerations November 2006
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Handley, et al. Informational [Page 38]